The invention relates generally to air flow monitors and more particularly to a temperature compensated air flow monitor for measuring true flow velocity or volume flow.
There are numerous applications for air flow monitors, such as spray booths, clean rooms, grinding operations and fume hoods. The monitored flow can be velocity flow or volume flow wherein volume flow is the area times the velocity through the area.
Fume hoods serve to protect an operator from noxious fumes contained or generated within the fume hood. The fume hood has one or more openings in a face of the fume hood and the exhaust is preset to provide a desired flow velocity of air into the hood. To assure the safety of the operator, it is necessary to confirm that the required flow velocity is maintained.
In general, prior art fume hood monitors measure the mass flow of the air rather than the true flow velocity. Such mass flow measuring systems, such as described in U.S. Pat. No. 4,548,128, appear to be dependent upon the density of the fluid or air flowing past the monitor. This does not necessarily relate to the true flow velocity of the fume hood.
A flow measuring system described in U.S. Pat. No. 3,942,378, states that the measuring system compensates for changes in fluid density. This measuring system however requires two separate complicated sets of flow sensors.
The thermal anemometer described in U.S. Pat. No. 4,537,068, states that true flow velocity is measured. The thermal anemometer includes a thermistor heated to a high operating temperature of 200.degree. C. Heating to this temperature utilizes too much power for practical portable applications. Further, the thermistor sensed temperature is corrected by a linear temperature device, which generally are expensive.
It would be desirable to measure the true flow velocity or volume flow and to provide temperature compensation for such measurement, for example, in a fume hood.